Collimation

[dawson]

When assessing how well collimated a scope is, using the method of taking a star out of focus and looking at how evenly the rings spread out around the central point, how big does one need to make the concentric rings? Should you make it as big as possible, and at what stage would one expect a well collimated scope to start to show asymmetrical concentric rings?

James

[xanthic]

If it's properly collimated you shouldn't get any asymmetry

[dawson]

Interestingly I just found an out of focus star on my cameras memory card.

 

It looks pretty symmetrical and concentric in this image, but if I keep going out of focus and get the circle really big, it then starts to become asymmetrical.

 

I assume collimation is a not "all or nothing", and that any optical system will fall within the range between "totally collimated to picometre accuracy" and "totally uncollimated".

 

I really want to know how the size of the out of focus star relates to the state of collimation. Is there a relationship or am I just assuming there is? I'd have thought any flaws in collimation would become more apparent the more the star is taken out of focus, but there isn't a Google result which supports this.

 

James

 

[Guest billywhizzzzz]

I have been wondering the very same , if I go say half a turn ( De focused ) Its not quite asymmetrical , however give it another full turn the same way , and its perfect .

Steve

[Brantuk]

You can always compare it against the results from using a bhatinov mask - use that to get it in focus then do a star test and see how the rings look.

[philjay]

A higher mag will help more than just defocusing to a big blob with a lower mag eyepiece. I always use my 8-24mm zoom which I bought for this very purpose when I had the C11 SCT. Start off at 24mm then tweak to get symmetry then up the mag and fine tune.

 

Its mag that counts in my experience and not just out of focus disc size.

[dawson]

Phil, the above image was with the DSLR at prime focus. I'll have to have a look visually with a high mag eye piece next time.

 

Kim, I'm not sure how the Bahtinov helps, as I am more interested in the star going out of focus and producing the rings rather than it being in focus; or am I missing the point.

 

James

[Al.Grant]

Hi,

 

Not sure if this will be of any use to you.   Al's Collimation Aid   (<== obviously this is a different 'Al' not me!)

It is basically a scalable overlay that you can line up over the air disc to highlight any slight collimation errors. Works on images or live webcam.

 

I had a quick look at it when checking the collimation of my VC200L.   I can't really judge how much of an advantage it is over an unaided eye as my collimation looked quite good, however I will be checking mine again as I've read that it's best to check the airy disc either side of 'in focus'  (at least that's the advice for the VC200L)

 

I have quickly put the overlay over the image you posted and it certainly looks close, but higher mag may show something and the seeing conditions would also play a big part.

Edited June 5, 2013 by Al.Grant

[Brantuk]

My thinking James is that the mask pattern will give you a precise starting point. So if you get the focus pattern on screen then you can be pretty sure the scope is collimated well. Then when you take it slightly in/out of focus for a star test you should see perfect concentric rings. I also like Phil's idea of using a zoom and adjusting from low to high magnification to refine collimation at each click stop.

 

One other thought is that the more of the eyepiece you fill with the rings, the closer you'll be to the edge which is where CA starts. I only go in/out of focus a small distance to keep the view in the center where most eyepieces perform best.

[dawson]

Al, that application looks interesting. I agree, my out of focus star looks pretty good. I will try and get some larger rings (more out of focus) and try again.

 

Kim, are you suggesting if the system achieves near perfect focus, then it is well collimated? I'm still not getting it; I'm very dim, sorry.

 

James

[Brantuk]

Yes if it can focus well as indicated by the bhatinov mask - then it will be near perfect collimation, which you can double check with the star test (and a zoom eyepiece). The mask is an indicator of perfect focus for imaging.

[dawson]

Seems like the test star must be dead centre else it will look like it is out of collimation:

 

http://www.company7.com/library/orion/Inst_makcasscollim.pdf

 

James

[andyboy1970]

I believe most say rock the focus either side of in-focus.

i.e. focus achieved at 10mm check at 5mm and 15mm, just an example.